In pressurized water reactors (PWRs), light water is used as the reactor coolant and neutron moderator. Electricity is produced by converting the light water into high-temperature and high-pressure water that does not boil throughout the primary system, generating steam by sending the high-temperature and high-pressure water to a steam generator and performing heat exchange, and sending the steam to a turbine generator.
In such a pressurized water reactor, the number of neutrons produced in the reactor core is adjusted by absorbing the neutrons by the use of control rods, whereby the output of the nuclear reactor is controlled. Accordingly, the control rods are dispersed and incorporated in advance in fuel assemblies forming the reactor core, and are collectively controlled. The control rods are inserted into and withdrawn from the reactor core by a control rod drive mechanism (CRDM) placed at the upper portion of the reactor vessel.
In general, a magnetic jack is often used as a control rod drive mechanism of a pressurized water reactor. A drive shaft of the magnetic jack is placed in a guide tube at the center. A stationary gripper magnetic pole, a stationary gripper spring, a stationary latch mechanism, and the like are formed at the lower portion of the guide tube. A stationary gripper coil is arranged near the stationary gripper magnetic pole. A lifting magnetic pole, a movable gripper spring, a movable gripper magnetic pole, a movable latch mechanism, and the like are formed at the upper portion of the guide tube. A lifting coil is arranged near the lifting magnetic pole, and a movable gripper coil is arranged near the movable gripper magnetic pole. Accordingly, the control rods can be driven vertically.
In this case, a coil used for the control rod drive mechanism must be cooled constantly because the temperature of the coil is increased when a current passes through the coil. Patent Document 1 discloses a cooling device for a control rod drive mechanism. In a cooling method for the control rod drive mechanism disclosed in Patent Document 1, ambient air is taken into a shroud along an upper portion of the shroud by a ventilation fan, and the air is discharged to the exterior through an air port at the lower portion of the shroud after cooling the control rod drive mechanism.    [Patent Document 1] Japanese Patent No. 2904410
In the conventional cooling method for the control rod drive mechanism described above, the control rod drive mechanism is cooled by ambient air forcibly directed into the shroud along the upper portion of the shroud with the ventilation fan and then directed to the control rod drive mechanism. The air that has cooled the control rod drive mechanism is further directed downward and is pushed out to the exterior through the air port at the lower portion of the shroud. In this case, with the cooling air flowing from above to below, it is difficult to discharge the air whose temperature has risen by cooling the control rod drive mechanism by further directing the air downward. Accordingly, the cooling air does not circulate efficiently in the shroud, resulting in reduced cooling efficiency of the control rod drive mechanism.
The present invention has been made to solve the problems described above, and an object of the present invention is to provide a cooling structure and a cooling method for a control rod drive mechanism and a nuclear reactor capable of improving the cooling efficiency of the control rod drive mechanism.